In an operator's mobile network, a Base Station (BS) communicates with multiple User Equipments (UE) within coverage. The communication between the BS and the UE is two-way. A process of transmitting signals to a UE by a BS is called downlink communication, while a process of transmitting signals to a BS by a UE is called uplink communication.
A duplex mode refers to a multiplexing mode between an uplink and a downlink. At present, duplex modes include simplex, half-duplex and full-duplex. In Simplex refers to the case that communication is one-way, where a transmitter is only capable of transmitting signals while a receiver is only capable of receiving signals, and signals can only be transmitted from the transmitter to the receiver. Half-duplex refers to the case that communication is two-way, but there is only uplink transmission or downlink transmission in a same transmission resource, where both sides of the transmission can not only transmit signals but also receive signals, but the transmitting and receiving of a same transceiver occurs at different transmission resources (time, frequencies, or orthogonal codes). Full-duplex refers to the case that a transceiver performs two-way transmission at a same transmission resource.
Particularly to a cellular network, communication between a BS and a UE is two-way, and all existing cellular communication systems and standards nowadays are half-duplex. According to various manners to divide uplink and downlink on transmission resource, cellular networks may be classified into two categories which are Frequency Division Duplexing (FDD) systems and Time Division Duplexing (TDD) systems. In a TDD system, an uplink and a downlink are distinguished by different timeslots. For example, in a Long Term Evolution (LTE) system, a frame is divided into an uplink subframe and a downlink subframe for uplink transmission and downlink transmission, respectively; and in general, to avoid interference between uplink and downlink, a protection subframe is introduced in TDD system when transitioning from a downlink subframe to a uplink subframe (a protection subframe may be not introduced when transitioning from an uplink subframe to a downlink subframe, because time for transition can be controlled by BS), and relative synchronization of the entire network is kept. FDD refers to the case that an uplink and a downlink are distinguished by different spectra; and in general, to avoid interference between uplink and downlink, a protection band is reserved between an uplink spectrum and a downlink spectrum in a FDD system.
Simultaneous transmission in uplink and downlink in a same time-frequency resource is achieved by full-duplex technology. Spectral efficiency of full-duplex is double of spectral efficiency of simplex and half-duplex. Current different antennae and radio-frequency channels are used by a transceiver of full-duplex technology for transmitting and receiving, for it has not been testified by an experimental prototype that a required result can be achieved by using a same antenna or radio-frequency channel. A problem needed to be solved by full-duplex technology is how to deal with interference on a received signal by a transmitted signal of a same transceiver. Here, the interference on a received signal by a transmitted signal of a same device is called self-interference.
A distance between a transmitting antenna and a receiving antenna is very small (usually no more than 10 cm), therefore, power of a transmitted signal received by a receiving antenna is very large. The very strong self-interference must be handled at an analog front end, otherwise the very strong self-interference will cause congestion at the analog front end (exceeding a linear range of a receiving amplifier and making the received signal smaller than a quantization precision of an analog-to-digital converter (ADC)). Self-interference between a received signal and a transmitted signal in full-duplex is shown in FIG. 1. With a path loss model of current macro base station as an example, L=128.1+37.6 log 10(R), where L indicates a path loss, i.e., attenuation of signal intensity with distance; R indicates a distance in unit of Km. A path loss from a terminal, which is 200 m far from a macro base station, to the macro base station is 102 dB, while a path loss from a transmitting antenna to a receiving antenna of a same transceiver is generally 40 dB. It is obvious that even in a case that the UE has the same power as the base station, self-interference of the base station is 62 dB stronger than a received uplink signal. Therefore, at present, research in full-duplex technology focuses on how to perform self-interference cancellation. At present, methods for self-interference cancellation include 3 aspects which are antenna, analog, and digital.
A full-duplex communication device corresponds to two different channels which are a self-interference channel from a transmitting antenna to a receiving antenna on a same communication device, and a transmission channel between two communication devices.
The self-interference channel is a frequency-selective channel which varies slowly with time. Between the transmitting antenna and the receiving antenna there is a multipath channel. In frequency domain, multi-path delay spread is small, so a multipath channel can be considered as a flat fading channel when bandwidth is small. However, the self-interference is very strong compared with a received signal, so accurate channel estimation is needed to accurately reconstruct interference, and then frequency density of reference signals used for self-interference channel estimation is required to be high. In time domain, multipath between a transmitting antenna and a receiving antenna of the same communication device mainly results from reflection and refraction in the body of the communication device, and positions of the transmitting antenna and the receiving antenna and a shape of the body are fixed, therefore, the channel in time domain vary slowly with ambient environment.
A transmission channel between two communication devices in a typical scene is a time-varying frequency selective channel. In frequency domain, multipath delay of a signal between two communication devices is large, so the channel response varies significantly in frequency domain and channel estimate needs to be performed with reference signals dense in frequency domain. In time domain, due to movement of the two communication devices or movement of ambient reflectors and refractors, the channel response varies significantly in time domain, therefore, reference signals dense in time domain are needed for channel estimation.
Channel responses of the two channels need to be estimated separately. When estimating a transmission channel, a full-duplex communication device leaves blank at a position where a communication device at the other end transmits a reference signal, so that a receiving path estimates according to the transmitted reference signal. When estimating a self-interference channel, the communication device at the other end needs to leave blank at a position where the local full-duplex communication device transmits a reference signal of the self-interference channel. Although at a side of the receiving antenna, intensity of the reference signal of the self-interference channel is much larger than that of a signal from another communication device, the signal intensity after self-interference cancellation is comparable with intensity of the received signal, therefore, it is necessary for the communication device at the other end to leave blank at the position corresponding to the reference signal.
In a full-duplex communication device, one communication device corresponds to multiple channels very different in time-varying characteristics. Reference signals with a same time domain interval are used to estimate each channel.
All existing standards are half-duplex communication, where there is no mutual interference between a transmitting antenna and a receiving antenna of a same communication device, so only reference signals for estimating a transmission channel response between two communication devices are designed in current standards.
The description, although made by taking a reference signal of an LTE system as an example, is certainly applicable to other Orthogonal Frequency Division Multiplexing (OFDM) systems/Orthogonal Frequency Division Multiple Access (OFDMA) systems.
In an LTE system, a minimum unit for transmitting time-frequency resources is a time-frequency resource element. Each time-frequency resource element is constituted of an OFDM symbol and a subcarrier. Channel estimation is performed by transmitting a known signal (a reference signal) on selected time-frequency resource elements dispersing in time domain and in frequency domain among available time-frequency resource elements. A frequency domain interval between reference signal elements is less than coherence bandwidth (inversely proportional to multipath delay spread) in a typical scattering scene, and a time domain interval is less than coherence time (inversely proportional to frequency domain Doppler spread) in a typical movement scene. A distribution of reference signals in an LTE system is shown in FIG. 2, where 1 indicates a timeslot.
In existing technology, reference signals are designed according to characteristics of time domain variation and frequency domain variation of a transmission channel between point-to-point communication devices. For a full-duplex communication device, it is required to estimate two channels (a self-interference channel and a transmission channel) at the same time and the two channels are very different in terms of characteristics of time domain variation and frequency domain variation. For the self-interference channel with frequency domain variation relation varying slowly with time, conventionally an amount of reference signals used for channel estimation is the same as that used for estimation of transmission channel, and time-frequency resource for data transmission is increased.